Conversion of heavy petroleum feedstocks

ABSTRACT

A MODIFIED FLUID COKING PROCESS IN WHICH PARTICLES COMPRISING CAO REPLACE THE COKE IN THE COKER, AND THE COKE DEPOSITED ON THE PARTICLES IS BURNED OFF IN THE BURNER. SULPHUR FROM THE COKE IS FIXED IN THE PARTICLES MAINLY AS CAS. SOME OF THE PARTICLES PASS INDIRECTLY BACK TO THE COKER VIA A REGENERATOR IN WHICH THEY ARE CONTACTED WITH OXYGEN WITH THE RELEASE OF SO2 IN USEFUL CONCENTRATIONS.

Dec. 26, 1972 5, oss

CONVERSION OF HEAVY PETROLEUM FEEDSTOCKS Filed Jan. 25, 1971 MW vm 1..

3,707,462 CONVERSION OF HEAVY PETROLEUM FEEDSTOCKS Gerald Moss, Oxford,England, assignor to Esso Research and Engineering Company Filed Jan.25, 1971, Ser. No. 109,124 Claims priority, application Great Britain,Jan. 27, 1970, 3,824/70 Int. Cl. C10g 9/28 US. Cl. 208127 14 ClaimsABSTRACT OF THE DISCLOSURE A modified fluid coking process in whichparticles comprising CaO replace the coke in the coker, and the cokedeposited on the particles is burned off in the burner. Sulphur from thecoke is fixed in the particles mainly as CaS. Some of the particles passindirectly back to the coker via a regenerator in which they arecontacted with oxygen with the release of S in useful concentrations.

The present invention relates to the conversion of heavy petroleumfeedstocks and more particularly to the conversion of such feedstockscontaining sulphur.

Heavy petroleum feedstocks such as residual are generally of low valueand they may be employed either alone or in blends with lighterfeedstocks as fuels, or they may be converted to products of greatervalue by various known processes.

One of the processes heretofore employed for the conversion of heavyfeedstocks is the process known as fluid coking wherein the feedstock issprayed, having been previously heated (if necessary) to facilitatespraying, into a bed of hot fluidized coke particles in a reactor. Abrief description of fluid coking processes is given in PetroleumRefinery Engineering, 4th edition, by Nelson, pp. 689-691. The feedstockis cracked into lighter products in the vapour phase and into coke, thecoke being deposited on the coke particles of the fluidized bed. Theproducts in the vapour phase are recovered and comprise normally liquidhydrocarbons which are of relatively higher value than the feedstock andwhich can be used for automotive and jet engine fuel use and/ormanufacture, together with normally gaseous hydrocarbons sometimes inadmixture with hydrogen, all of which are useful products or feedstocksfor further processing e.g. for the manufacture of olefins.

The particles of coke are circulated from the reactor to a burnerwherein they are fluidized in an oxygen-containing gas and therebypartially combusted and raised in temperature, some of the thus heatedcoke particles being returned to the reactor for futher use, theremainder of the coke being withdrawn as a by-product. In a typicalfluid coking unit, the feedstock is converted to about 70% of normallyliquid products and about 25% of coke, and 8% of the latter is consumedin the burner to provide heat for the process.

When the feedstock contains sulphur and other materials such as vanadiumand sodium which are generally regarded as deleterious contaminants, thecontaminants tend to be more concentrated in the coke than in the usefulvapour phase products recovered from the reactor, and hence, the gasesleaving the burner have high concentrations of sulphur, while the cokeby-product is high in sulphur and in other contaminants. As a result,the burner off-gases are not very suitable for further use since if thecarbon monoxide content is employed to produce heat, the resulting fluegases are polluted with sulphur oxides, while if they are employed toproduce synthesis or industrial gases, the sulphur content tend todeactivate United States Patent 0 3,707,462 Patented Dec. 26, 1972 ICCwater-gas shift catalysts. Similarly, the uses to which the coke may beput are restricted by the presence of contaminants.

The present invention provides a method of converting asulphur-containing heavy petroleum feedstock comprising the steps ofpassing the feedstock into a first bed of fluidized particles comprisingcalcium oxide or a precursor thereof at a temperature between 500 and700 C. whereby the feedstock is converted to vapours comprising normallyliquid and gaseous products of reduced sulphur content and tocarbonaceous material of increased sulphur content which deposits on thesaid particles, recovering the said vapours from the first bed and transferring particles from the first bed to a second bed wherein theparticles are fluidized at a temperature of 800 C. to 1000 C. in a gascontaining oxygen in an amount suflicient to convert at least some ofthe carbonaceous material to gases containing a carbon oxide whereby thecarbonaceous deposits on the particles are at least partially removed,and at least some of the sulphur originally present in the carbonaceousdeposits reacts with the calcium oxide to form calcium sulphide,recovering the gases containing at least one carbon oxide from thesecond bed, transferring some of the particles from the second bed tothe first bed and transferring particles from the second bed to a lowerzone of a third bed in which the particles are fluidized at atemperature of 1000 to 1100 C. in an oxygen containing gas whereby atleast some of the calcium sulphide in the particles is converted tocalcium oxide with the release of S0 and transferring particles from anupper zone of the third bed to the first bed.

Thus, it will be appreciated that a heavy petroleum feedstock,contaminated as aforesaid, can be converted to relatively uncontaminatedlighter products in the first bed, with the production in the second bedof substantially sulphur-free off-gases suitable for conversion by thewater-shift reaction to substantially pure hydrogen, blue water gas,synthesis gases for ammonia manufacture or for Fischer-Tropschreactions, the type of ofi-gas depending on the nature of theoxygen-containing gas supplied to the second bed. The oil-gases from thethird bed contain S0 in proportions which, with suitable conditions ofoperation of the process, are sufiiciently high for the S0 to be usefulrather than a pollutant.

The temperature in the third bed is maintained in the range 1000 to 1100C., preferably l050-1070 C., by sensing the temperature in the third bedand increasing the rate of transfer of particles from the cooler secondbed when the tmperature in third bed increases, and decreasing the rateof transfer when the temperature decreases.

The reaction Which take place in the third bed leading to the evolutionof S0 can be represented as follows:

The overall reaction is exothermic and at temperatures of 1000 to 1100(1., almost all of the sulphide which reacts is converted to oxide, theremainder being converted to sulphate which may be reduced to the oxideon a subsequent pass through the third reactor.

In order to ensure that the S0 content of the off-gases from the thirdreactor is at a usefully high level and not diluted by unreactedoxygen-containing gas, the off-gases may be monitored for S0 or oxygen,and if the concentration of S0 falls or the oxygen concentrationincreases, the rate of supply of the oxygen containing-gas to the thirdreactor is reduced accordingly. Theoretically, the S0 concentration canbe 15% When the oxygen-containing gas is air, but 10-12% is usuallyachieved in operation, and the S at this level of concentration isuseful for the manufacture of pure S0 sulphuric acid and for reductionto sulphur.

The activity of the particles to fix sulphur as calcium sulphide forsubsequent release as S0 from the third reactor tends to diminish as theparticles are recycled, and it is preferred to bleed off some particleseither continuously or intermittently to maintain activity. Preferably,the particles are bled off and dumped from the bottom zone of the thirdreactor in response to a signal indicating that the sulphur-trappingactivity has diminished or is insufficient. A suitable indication ofsufiicient activity is obtained by monitoring the sulphur content of atleast some of the vapour(s) recovered from the third bed: forconvenience, it is preferred to monitor the H 8 content of the vapours,either before or after normally liquid products have been condensedtherefrom since the H content of the vapours will be related to thesulphur content of the feedstock and of the various products recoveredfrom the first reactor. A substantially constant inventory of activeparticles circulating in the three reactor beds must be maintained, andthis may be achieved by sensing the amount of particles in the first bedand adding fresh particles to the second (and/or third) bed(s) inaccordance with any defficiency of particles sensed in the first bed.

The temperature and oxidising potential in the second bed must be somaintained that the carbonaceous deposit is substantially removed fromthe particles therein without causing sulphur to be lost from theparticles. A suitable method of maintaining the oxidizing potentialbelow the level for converting calcium sulphide to sulphate or tocalcuim oxide (which reactions occur to any extent only when theoxidizable deposit has been removed) is to monitor the carbon dioxideconcentration in the off-gases from the second bed, and varying the rateof supply of oxygen to the bed in accordance with the variation in COcontent of the off-gases. The CO concentration is preferably maintainedat less than 75% of the maximum concentration which can be achieved bystoichiometric combustion of the carbonaceous deposit. Thus, if theoxygen containing gas supplied to the second bed is air, the COconcentration is preferably less than 11% and more preferably less than8%, the remainder of the off-gas being carbon monoxide andnon-combustible components of the oxygen. When the CO concentrationexceeds a predetermined maximum concentration (according to thecomposition of the oxygen-containing gas), the rate of supply of oxygencontaining gas is reduced until the CO concentration in the off-gas isreturned to or tending towards a satisfactorily low concentration.

Generally speaking, the amount of carbonaceous deposit on the particleswill be related to the Conradson carbon number of the feedstock to thefirst bed, and accordingly, the rate of supply of oxygen to the secondbed may be made generally proportional to the rate of supply offeedstock to the first bed, the proportionality factor taking intoaccount the Conradson carbon number of the feedstock.

The oxygen containing gas supplied to the second bed may be arranged tohave a composition such that the offgases are useful by-products orintermediates. Thus, the gas may be air enriched with oxygen such thatthe off-gases will contain C0, C0 and N in proportions suitable for theapplication of the water gas shift reaction:

to provide, after washing out the CO by methods known in the art, theremaining gases containing nitrogen and hydrogen in suitable proportionsfor ammonia synthesis. Another suitable oxygen containing gas comprisesa mixture of oxygen and steam so that the off-gases from the second bedconsist largely of hydrogen, CO and CO this type of off-gas can bescrubbed to remove CO leaving a gas suitable for Fischer-Tropschsynthesis reactions, or it may be mixed with steam and passed over awater-gas shift catalyst to provide, after scrubbing out the resultingCO substantially pure hydrogen. In the instances so far mentioned, theamount of oxygen in the oxygen-contain ing gas can be varied inaccordance with known thermo dynamic principles and in accordance withthe amount 01 combustible deposit on the particles to give the desiredtype of off-gas. The off-gases may be used for heat produc tion bycombustion instead of intermediates for further processing.

It is important that the temperature in the second bed is maintained inthe range 8001000 C. to ensure that sulphur or sulphur compounds are notevolved: the presence of sulphur or its compounds would graduallyinactivate any water gas shift catalyst. Accordingly, it is preferred tosense the temperature in the second bed, and when the temperature risesabove a selected temperature, to inject a diluent into the second bed.Suitable diluents are water, steam and nitrogen, and the amount ofdiluent injected is preferably varied as the bed temperature varies.

It is also important that the temperature in the first bed is maintainedbetween 500 and 700 C., below 500 C., the rate of conversion offeedstock is relatively uneconomic while above 700 C., the amount ofcracking of feedstock to light hydrocarbons and hydrogen and the amountof carbon deposit on the particles become excessive. Accordingly, if therate of supply of feedstock to the first bed is substantially constant,the temperature in the first bed may be sensed, and the rate of transferof particles from the first bed to the second bed decreased when thetemperature in the first bed rises and decreased when the temperature inthe first bed decreases: thus the amount of carbonaceous materialtransferred from the first bed to the second bed for combustion leadingto heat generation is accordingly restricted, and the heat return to thefirst bed is decreased. Alternatively, the feedstock supply rate may beincreased with an increase in temperature and decreased with a decreasein temperature, but this is less convenient for most refineryoperations.

The amount of particles in the second bed is preferably maintainedwithin a chosen range sufiicient for a particle having an average sizeand an average deposit of carbonaceous material thereon to have anadequate residence time for removal of at least a major portion, orpreferably substantially all, of the carbonaceous deposit. The residencetime will, of course, depend on the design of the vessel containing thesecond bed, and the amount of particles in the second bed is maintainedin the chosen range by sensing the amount in the bed (e.g., by staticpressure determinations) and transferring particles to the first bed ata rate which increases with any increase in the amount in the secondbed, and decreases with any decrease in the amount in the second bed.

The particles in the first bed may be fluidized not only by thevapourous conversion product, but also by a suitable fluidizing gas orvapour. For this purpose, steam is preferred since it is relativelycheap and can be removed as water from the products. The steam also actsto strip any volatile materials trapped in the deposit on the particles.

The invention also includes the various products made by the foregoingmethod. It will be appreciated that the invention enables potentialpollutants and waste materials to be converted to commercially usefulproducts and intermediates.

An embodiment of a plant for performing the invention is illustrated byway of non-limitative example in the accompanying drawing, the plantbeing shown semischematically.

The feedstock, such as a residuum containing sulphur and otherundesirable materials, such as sodium, vanadium and iron, is suppliedvia line 10 to a fluid coker vessel 11 in which there is contained a bed12 of fluidized particles comprising lime (e.g. calcined limestone orcalcined dolomite). The rate of supply of the feedstock is measured by ameter 13, and steam is injected into the base of the coker 11 from line14 at a rate which is correlated with the fuel supply rate by means of asuitable meter 15. The temperature in the bed 12 is maintained in therange 500700 C. and in this range, the feedstock is converted tovapour-phase products, and carbonaceous materials such as coke and heavytars. The coke and tarry materials are deposited on the fluidizedparticles while the vapour-phase products are carried upwards out of thecoker 11 via a cyclone 17 and a slurry trap 16 to a fractionator 18. Thesulphur content of the original feedstock tends to be more concentratedin the coke and tarry materials than in the vapour phase products, as doother contaminants such as vanadium and sodium.

In the slurry trap 16, heavy hydrocarbon materials admixed with finesfrom the bed 12 are trapped, and are recycled via line 19 to the bed 12,possibly, but not necessarily, in admixture with the feedstock in line10.

The coked and tarry particles move towards the bottom of the coker 11,and any volatile materials trapped thereon are stripped off by the steamfrom line 14. The coked particles are withdrawn via line 20 and passedto a second bed 21 in a burner vessel 22. The bed 21 contains particlesfluidized in an oxygen containing gas which is supplied from line 23 bymeans of fan 24 at a rate controlled by a valve 25. The oxygencontaining gas may be air, oxygen-enriched air, air and steam or steamadmixed with substantially pure oxygen.

In the burner bed 21, the carbonaceous deposit on the particles isremoved either wholly or to a major extent with the generation of heat.The sulphur content of the deposit is trapped by reaction with thecalcium oxide of the particles as calcium sulphide, and the sodium andvanadium also are retained by the particles: the sodium and vanadiumretention of the particles is sometimes improved if iron is present,either as an impurity picked up from the original feedstock or as animpurity in the particles.

The temperature in the bed 21 must be maintained between 800 and 1000 C.to ensure an adequate minimum temperature for satisfactory combustionand a suitable maximum temperature to prevent the decomposition of thecalcium sulphide with the evolution of sulphur or sulphur compounds. Tomaintain the temperature within these limits, the maximum rate of supplyof oxygenated gas from the fan 24 is slightly greater than would benecessary for the highest rates of carbon and coke lay-down on theparticles from the feedstock, the actual supply rate to the bed 21 beingregulated by the valve 25. A temperature probe 26 senses the temperaturein the bed 21, and as the temperature tends towards 1000 C., signalsfrom the probe 26 cause a valve 27 to open progressively therebyallowing a regulated amount of diluent from line 28 into the bed 21. Thediluent is preferably steam, but nitrogen may be used in addition oralternatively.

The amount of particles in the bed 21 is determined by pressuretransducers 29, 30 from which a differential pressure signal is derivedby a differential pressure manometer 31. As the differential pressureapproaches a maximum desired value, a signal from the manometer 31causes a control valve 32 to open progressively, thereby allowing anincrease flow-rate of a propelling fluid from a line 33 to the maintransfer line 34. The propelling fluid is preferably steam, and itentrains, fiuidizes and propels particles along the smoothly curvingtransfer line 34 from near the base of the burner vessel 22 to thecoking vessel 11, the particles entering near the top of the bed 12.

The gases leaving the burner 22 pass through a cyclone 35 to a take-offline 36 and are monitored for CO by an infra-red monitor 37. If the COcontent of the off-gases is excessive indicating excessive oxidizingconditions within the bed 21, a signal to the controller 38 on valve 25causes the valve 25 to be progressively closed until the COconcentration is acceptably low. The maintenance of a low COconcentration in the off-gases ensures that minimal quantities ofcalcium sulphide in the burner bed 21 are converted to other compounds,possibly with the loss of sulphur in the off-gases from the burner.

Some particles are transferred from the burner bed 21 via a smoothlycurving line 39 to the lower region of a third bed 40 of a regeneratorvessel 41 wherein the particles are fluidized above a distributor 42 inan oxygencontaining gas, such as air, supplied by a fan 43. Theparticles in the bed 40 should be substantially free of carbon, and atthe temperatures of 1000 to 1100 C., the calcium sulphide is convertedto calcium oxide with the evolution of S0 and the production of heat.The SO containing off-gases leave the bed 40 via a cyclone 44 and aremonitored either for oxygen or S0 by a detector 45. If the S0 content ofthe gases is too low, the signal received from the detector 45 by avalve controller 46 will cause a valve 47 in the air supply line 48 toclose until the designed S0 concentration is attained. An S0concentration in the range 9% to 11% should be attainable. The detector45 can be equally used to monitor oxygen: provided the free oxygencontent of the off-gases does not exceed 1%, the S0 content will besatisfactory, and probably in the range 9% to 11%.

Control of temperature in the regenerator bed 40 is achieved by means ofa temperature transducer 49 which acts on the controller 50 of a valve51 in a steam propellant line 52. As the temperature in the bed 40increases, the valve 51 is progressively opened, thereby increasing therate of supply of particles from the relatively cooler bed 21 to therelatively hotter regenerator bed 40, thus reducing the temperaturetherein.

In order to prevent an excessive accumulation of particles in the bed40, particles are transferred from a top region of the bed 40 to thecoker bed 12 along the smoothly curving line 53 by steam propellantsupplied via line 54. The rate of steam supply along line 54 isregulated by control 55 which opens progressively as the static pressurein the bed 40 increases: the static pressure is measured by transducers56.

It will be appreciated that the particles returned to the coker bed 12will be considerably hotter than the temperature desired in the bed 12.Most of the heat thus supplied to the coker bed 12 is employed for theactual conversion process which is endothermic. Temperature control inthe coker bed 12 is achieved by sensing the temperature using transducer58, and employing the resulting temperature signal to control the supplyof propellant stream from line 59 to the smoothly curving transfer line20, so that particles are transferred at a decreased rate as thetemperature in the coker bed 12 increases and less particles are therebytransferred to the burned bed 21 and regenerator bed 40, less heat thusbeing generated. If the temperature in the coker bed 12 decreases, therate of transfer of particles to the burner and regenerator beds isincreased. This method of temperature control is applicable when thefeedstock supply rate is substantially constant and since for mostrefinery operations, the coker would be run at about its designed loadconditions, this would be the most economical method of temperaturecontrol. However, if it is economically possible to vary the feedstocksupply rate, an increase in the coker bed temperature can be sensed andemployed to cause an increase in the feedstock supply rate, whereby dueto the endothermic nature of the conversion reaction, the coking bedtemperature will fall. Similarly, a decrease in coker bed temperaturecan be employed to cause a decrease in the feedstock supply rate. Inthis type of temperature control method, it is not necessary to vary therate of supply of particles to the burner bed, although some variationmay be made if desired.

The sulphur-fixing activity of the particles is maintained by bleeding01f particles through a dump valve 60 from a bottom (oxidizing) regionof the bed 40. The regulation of the dump valve 60 is in accordance witha signal from an H S monitor 61 which monitors the gases (gaseoushydrocarbons and hydrogen) leaving a condenser 62.

A substantially constant inventory of active particles is maintained inthe system as a whole by monitoring the head of particles in the bed 12by means of transducers 63, 64 and using the differential transducer 65to control the opening of a bell-mouth lock hopper 66 via a relay 67.Fresh lime, limestone or dolomite particles are introduced when the lockhopper 66 is opened.

It will be apparent to those skilled in the art that the examplary plantdescribed and illustrated can be modified for performing the inventionwithout departing from the ambit of the appended claims.

I claim:

1. A method of converting a sulphur-containing heavy petroleum feedstockcomprising the steps of passing the feedstock into a first bed offluidized particles comprising calcium oxide or a precursor thereof at atemperature between 500 and 700 C. whereby the feedstock is converted tovapours comprising normally liquid and gaseous products of reducedsulphur content and to carbonaceous material of increased sulphurcontent which deposits on the said particles, recovering the saidvapours from the first bed and transferring particles from the first bedto a second bed wherein the particles are fluidized at a temperature of800 C. to 1000 C. in a gas containing oxygen in an amount sufiicient toconvert at least some of the carbonaceous material to gases containing acarbon oxide whereby the carbonaceous deposits on the particles are atleast partially removed, and at least some of the sulphur originallypresent in the carbonaceous deposits reacts with the calcium oxide toform calcium sulphide, recovering the gases containing at least onecarbon oxide from the second bed, transferring some of the particlesfrom the second bed to the first bed and transferring particles from thesecond bed to a lower zone of a third bed in which the particles arefluidized at a temperature of 1000 to 1100 C. in an oxygen containinggas whereby at least some of the calcium sulphide in the particles isconverted to calcium oxide with the release of S and transferringparticles from an upper zone of the third bed to the first bed.

2. A method according to claim 1 in which the temperature in the thirdbed is sensed and the rate of transfer of particles from the second bedto the third bed is increased and decreased with respective increasesand decreases in the temperature in the third bed.

3. A method according to claim 1 in which the oxygen content of theoff-gases leaving the third bed is sensed, and the rate of supply of theoxygen containing gas to the third bed is increased and decreased withrespective decreases and increases of the oxygen content of saidofi-gases.

4. A method according to claim 1 in which the S0 content of theolT-gases leaving the third bed is sensed, and the rate of supply of theoxygen containing gas to the third bed is increased and decreased withrespective increases and decreases of the S0 content of said offgases.

5. A method accoi ding to claim 1 in which the sulphur content of atleast some of the vapours recovered from the first bed is determined andwherein a portion of particles are dumped from the third bed, saidportion being increased with respective increases in said sulphurcontent.

6. A method according to claim 1 in which the static head of particlesin the first bed is sensed, and whenever the static head falls below aselected static head, fresh particles are added to the second bed in anamount sufficient to maintain the static head in the first bed at theselected static head.

7. A method according to claim 1 in which the ofi-gases leaving thesecond bed are analyzed for CO and the rate of supply of oxygen to thesecond bed is increased and decreased with respective decreases andincreases of the CO content of said off-gases.

8. A method according to claim 1 in which the rate of supply of oxygencontaining gases to the second bed is increased and decreased withrespective increases and decreases of the rate of supply of feedstock tothe first bed.

9. A method according to claim 1 in which the oxygen containing gassupplied to the second bed comprises air enriched with additional oxygenin such proportions that the gases leaving the second bed have a contentof C0, C0 and N suitable for subjecting to a water gas shift reaction.

10. A method according to claim 9 in which the gases leaving the secondbed are mixed with steam and subjected to a water gas shift reaction toproduce a feedstock for the production of ammonia.

11. A method according to claim 1 in which the oxygen containing gassupplied to the second bed comprises oxygen and steam in suchproportions that the gases leaving the second bed have a content ofcarbon monoxide and hydrogen suitable for reactions selected from watergas shift reactions for the production of hydrogen, and Fischer-Tropschsyntheses.

12. A method according to claim 1 in which the temperature in the secondbed is sensed and diluent selected from nitrogen, water and steam issupplied to the second bed when the temperature in the second bed is inexcess of a required temperature, the amount of diluent varying inaccordance with the excess of the temperature relative to the requiredtemperature.

13. A method according to claim 1 in which the temperature in the firstbed is sensed and the rate of transfer of particles from the first bedinto the second bed decreased When the temperature in the first bedincreases and increased when the temperature therein decreases.

14. A method according to claim 13 in which the amount of particles inthe second bed is determined to maintain a predetermined amount therein,and the rate of transfer of particles from the second bed to the firstbed increased and decreased with respective increases and decreases inthe amount of particles in the second bed relative to the predeterminedamount.

References Cited UNITED STATES PATENTS 2,348,543 5/1944 Johnson 2082262,364,390 12/1944 Schaafsma 208226 2,824,047 2/1958 Gorin et al 201-173,194,644 7/1965 Gorin et al. 48-197 R 3,402,998 9/ 1968 Squires 2082263,481,834 12/ 1969 Squires 208127 JAMES E. POER, Primary Examiner A. P.DEMERS, Assistant Examiner U.S. Cl. X.R.

